1. Field of the Invention:
The present invention relates to a stencil printing apparatus for carrying out printing by using stencil sheet, particularly to a stencil printing apparatus capable of immediately carrying out normal printing even when the apparatus has been left to a nonprinting state for a long period of time.
2. Description of the Related Art
In the printing operation of a stencil printing apparatus, a stencil sheet perforated by a perforating section is wrapped around a drum and thereafter, a sheet supply section supplies a print sheet to the drum. A press roller presses the print sheet to the drum to thereby carry out the printing and the printed sheet is discharged to a sheet discharging section.
According to such a stencil printing apparatus, when the apparatus is left without carrying out the printing operation for a long period of time, ink at inside of the drum or on the surface of the drum is dried or deteriorated. Therefore, when the printing operation is newly started after the apparatus has been left to the nonprinting state, there causes a problem in which image blur on a print sheet or a dispersion in print density occurs and a number of incomplete printed sheets referred to as "failure printed sheets" are discharged during a time period until normal printing is carried out in which an image is uniformly printed on the print sheet. The problem increases the printing cost and the printing time.
In order to resolve the above-described problem, according to an apparatus disclosed in Japanese Unexamined Patent Publication No. 3-175081, there is provided a mode referred to as "automatic idling mode".
According to the mode, a time point at which the perforating operation and printing operation have finally been carried out is memorized to the apparatus and then a time interval between the memorized time point and a current time point is calculated in the next perforating operation. When the calculated non-operational time period is equal to or more than a set value of the automatic idling mode, nonperforated master sheet is wrapped around the drum, a pressing operation the same as that in printing is carried out without supplying sheet by which ink is made adaptable.
According to the above-described constitution, when one drum unit is used for one apparatus, the function is sufficiently achieved and the failure can be prevented. However, when a plurality of drums is used for one machine, although the automatic idling mode is operated for a drum unit which is initially used, when the drum unit is interchanged by another drum unit, the automatic idling mode is not operated to the interchanged drum unit and failure occurs in order to obtain proper printed matter.
For example, three drum units holding respectively different inks are used to obtain printed matter having three colors. First, a predetermined number of print sheets are printed by a first color ink in a state in which a drum unit for the first color is mounted to the apparatus, thereafter, the drum unit of the first color is interchanged by a drum unit of a second color and the print sheet which has already been printed with the first color is overlappingly printed by second color ink. Finally, the drum unit of the second color is interchanged by a drum unit of a third color and the print sheet which has already been printed with the two colors is overlappingly printed by third color ink.
In such a case, the drum unit of the first color stays to be mounted to the apparatus. The pertinent information of the non-operational time period since the use at a preceding time can be provided and accordingly, the auto idling operation can be carried out in respect of the drum as necessary, so that failure can be prevented.
However, information of the non-operational time period for the drum unit of the second color, which is substituted for the drum unit of the first color and is mounted to the apparatus, is calculated with a time point of finishing the printing operation by the first color drum as a reference and the auto idling operation is not carried out when the printing operation is carried out without interposing a time interval and accordingly, failure occurs in the printing operation by the second color. Also in respect of the drum unit of the third color, similar to the case of the second color, failure occurs.
Hence, when a plurality of drum units is used to interchange in multiple color printing or the like, the printing operation is obliged to carry out by a larger number of printed sheets by previously anticipating failure of the second color and the third color, which consumes labor, time and cost.
The present invention has been carried out in order to resolve the above-described problem and it is an object thereof to provide a stencil printing apparatus producing printed matter having no image blur or a dispersion of print density without causing failure even when the apparatus is used by interchanging drums.